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Galactic Center
The core of each galaxy is different. Usually, it is a crowded place, with many stars, with a supermassive black hole in its center. As for today, the way galactic centers are formed and evolve is not well understood, as theoretical models don't match with observed data. Also, since galactic cores are very bright and often surrounded by dust and gas, there is not enough data available. Anyway, settlers that one day will come there will be more interested in what opportunities they can find and not in what was the distant past. In the following, we will use the core of the Milky Way as an example, because it is the most studied galactic center. However, as one can see, there are significant differences between each galaxy. Structure First of all, in the center of each galaxy lies a supermassive black hole. Some scientists suggest that there might be galaxies with a central black hole, however, this should be something very rare. Surrounding it, there is a halo or a disk of stars, dust and gas. Accretion disk. This is found in case of Quasars, which are active supermassive black holes. Inside the accretion disks, matter gets squeezed, becomes bright and starts fusion. Stellar halo. In some galaxies, including the Milky Way, stars in the core have rather chaotic orbits and form a bright sphere. Stellar disk. In other galaxies, the spiral structure continues deep in the core. In this case, stars are not in a too large number. Void. At least in theory, in case of a Silent Galaxy, all matter in and surrounding the core has been used for creation of stars, which, in most cases, ended-up their lifetime. A significant number of stars had been swallowed by the black hole. So, stellar population, mostly made by M - type stars, is small. Circuit of matter In the galactic center, star formation is accelerated, compared to other parts of a galaxy. Because of this, there are far more stars, including some very bright ones, which last little. All gas should be used fast for stellar formation. However, it looks like there is a process that slowly brings matter from outside, allowing for more stars to form. Some suggest that stellar formation occurs in bursts. In theory, when a cloud of gas and dust approaches the central black hole, it will break apart and will form many stars. Depending on how fast this movement is, stars might live for long enough, might be short lived (and swallowed by the black hole) or might not have even time to form completely. Core of the Milky Way In the core of the Milky Way, in the central parsec (a cube with radius of 3.26 light years), there are 10 million stars. This accounts for roughly 300000 stars in a cubic light year. Average distance between stars is around 900 AU. There are all types of stars. The vast majority, like anywhere, is represented by M - type stars or red dwarfs, but also the very bright O - type stars, Wolf - Rayet Stars or Red giants. Very interesting is that many stars seem to have very strange orbits, tilted or extremely elliptic. As they get very close to the supermassive black hole, their orbital speed increases dramatically. By opposite, in spiral galaxies, stars tend to have more circular orbits. Habitable places It is theorized that galactic cores were quasars in the beginning of galactic formation. A quasar produces a very bright and powerful light, which can sterilize any surrounding planet. However, this process does not last forever. In case of a mature galaxy like the Milky Way, such bursts occur when something falls in and are not so powerful and followed by periods without activity. In the central parsec of the Milky Way, there are at least 100 bright Wolf - Rayet, O - type stars and red giants. Based on their high number, supernova events occur at roughly each 10000 years. This also affects existing planets. One major problem for planets is that they are disturbed from their initial orbits. As a star gets very close to the supermassive black hole or to another star, its planets have their orbits affected. The galactic center is not a place for Kuiper belts and Oort clouds. The bright light from all surrounding stars will provide enough heat for many volatiles to sublimate. Stable orbits for a planet might exist only very close to the star. For M - type stars, their Habitable Zone is very close to the star, where stable orbits should be. Also, there should be many free- floating planets. A Rogue planet could be terraformed without the need of an Artificial sun. With so many bright stars surrounding it, there should be enough light, so that, with little use of Greenhouse Gases, the planet would become habitable. Colonization The first to come will be scientists, which will like to explore this unique environment. Then, there will come settlers. Planets in the galactic core will have limited resources of water and almost no atmosphere. Settlers could bring what they need to terraform these worlds from other places of the galaxy, if this proves to be feasible. however, newly created planets could have more volatiles on their surface and could be terraformed more easily. Industrial colonization could be an option. With a violent past of many supernovas, there could be high amounts of metals, including rare earths and actinides. The fact that there will be less water and light elements will also help space mining to develop. A more advanced civilization will look for something else. The central black hole can provide enough energy for a whole galactic empire. This energy can be produced by letting matter to fall in. There are theories that propose direct extraction of energy with a very powerful magnetic system. As for now, such technologies are not attainable.